This invention relates generally to a method and apparatus for generating a linear air/fuel signal and, more particularly, to an air/fuel sensor system for an internal combustion engine and a method of operating the system in time shared voltage sensing/current driving modes.
Presently, exhaust gas sensors switch to indicate when the air/fuel mixture is rich or lean. One such sensor is the Heated Exhaust Gas Oxygen Sensor (HEGO) which has a logarithmic voltage output versus air/fuel mixture and is essentially used as a switch to indicate if the mixture is rich or lean. The HEGO has one cell and only measures the voltage across the sensor, without pumping oxygen ion current through the sensor. While the HEGO sensors normally provide a very accurate indication of rich or lean variations from stoichiometric engine operation, they cannot accurately indicate the extent of rich or lean operation of an engine as it departs from stoichiometry.
The Universal Exhaust Gas Oxygen Sensor (UEGO) does provide the desired linear signal. The sensor, however, is more expensive to produce since it has two cells and is constructed of two chambers. One cell operates as a pumping cell, wherein current applied to the cell is controlled to pump oxygen between the chambers. The resulting current can be positive or negative and will indicate both rich and lean mixtures. The second cell operates as a sensing cell to generate the sensor signal. However, the UEGO tends to have a shortened life at high exhaust manifold temperatures compared with the simpler construction of the HEGO.
One reference which discloses operation of an air/fuel sensor in a time shared manner is U.S. Pat. No. 4,716,760. This patent discloses a tubularly shaped structure comprising a solid electrolyte coupled with a diffusion resistor for measuring the air/fuel ratio of an internal combustion engine. A predetermined current is supplied to the solid electrolyte for a predetermined period of time to pump oxygen into the tubular structure the diffusion resistor. A predetermined current of the opposite polarity is then supplied to the solid electrolyte for a sufficient period of time to pump the oxygen out of the tubular structure. The time required to deplete the oxygen in the tubular structure at the boundary between the diffusion resistor and the solid electrolyte is signaled by the voltage generated across the solid electrolyte due to the difference in oxygen component pressures between the sides of the solid electrolyte. The biasing and depleting operations are performed in a repetitive manner referred to as a time sharing manner. However, the time sharing is between pumping oxygen in one direction to bias the diffusion resistor and then in the opposite direction to deplete the oxygen from the diffusion resistor.
Hence, it would be desirable to provide an air/fuel sensor system which is capable of providing an indication of the extent of rich or lean operation of an internal combustion engine as it departs from stoichiometry. It would further be desirable to provide a sensor having a linear output like the UEGO, but with the simplified construction of the HEGO.